a. Field of the Invention
The invention relates to recording media, and more particularly to a reflective silver data recording and storage medium useful for reading laser recordings immediately after laser writing.
b. Prior Art
Previously, many types of optical recording media have been developed for laser writing. For example, an article in Optical Engineering, Vol. 15, No. 2, March-April, 1976, p. 99 discusses properties of a large number of media. Some of these media require post write processing before they can be read, and some can be read immediately after laser writing. The media of interest herein are for "direct read after write" capability, commonly known as "DRAW" media. Presently known laser DRAW media are thin metal films in which holes may be melted, composite shiny films whose reflectivity at a spot may be reduced by evaporation, thin films of dyes or other coatings which can be ablated at a spot, and dielectric materials whose refractive-index may be changed at a point, causing a scattering of light when scanned with a read laser.
The most common DRAW media are thin metal films, usually on a glass substrate. Thin metal films have several advantages: First, they can be produced for research purposes in small quantities with commercially available sputtering equipment. Second, they can be read either by reflection or by transmission. Third, films of tellurium and bismuth have relatively high recording sensitivities.
Fortunately, for all of these reasons, metal films have enabled a large amount of research to be conducted and progress to be made in the design of optical data storage systems. To date, tellurium and amorphous mixtures thereof have evolved as the most widely used of the metal films. However, tellurium must be manufactured by a relatively expensive, batch-type, vacuum sputtering technique; it does not form a tenacious coating; and it introduces manufacturing and environmental complications because of its toxicity and since it rapidly oxidizes in air it must be encapsulated in an airtight system in order for it to achieve an acceptable archival life. It has been reported in the literature that by forming amorphous tellurium mixtures with arsenic and selenium the rate of oxidation is reduced.
What is particularly desirable about tellurium is that it has a low melting temperature for a metal, 450.degree. C., and also a very low thermal conductivity of 2.4 watts per meter per degree Kelvin at 573.degree. K. In comparison, silver metal has a melting temperature of 960.degree. C. and a thermal conductivity of 407 watts per meter per degree Kelvin at the same elevated temperature. When these two metals are considered for laser recording with short pulses of laser power, the tellurium is far superior from a recording sensitivity standpoint since the low thermal conductivity keeps the heat generated by the laser beam confined to a small area and the lower melting temperature facilitates the melting of the hole. Conversely, silver metal, because of its high thermal conductivity, about 170 times that of tellurium, would not normally be considered suitable for laser recording.
Attempts have been made to improve the laser recording sensitivity of various types of metal layers. In U.S. Pat. No. 3,911,444 Lou, Watson and Willens disclose a vacuum-deposited metal film recording media for laser writing incorporating a separately deposited plastic film undercoat between the metal film and a flexible transparent substrate to thermally insulate the metal layer in order to require less energy to write with a laser.
A surface can reflect large percentages of the incident light without being electrically conductive. It is known that if very small, electrically conductive metal spheres or spherical particles are distributed through a dielectric medium, the effective dielectric constant or refractive index will rise owing to the added induced dipoles of the metal particles. For the case of homogeneously distributed particles, see Principles of Microwave Circuits, edited by C. G. Montgomery, McGraw Hill Book Company, Inc., 1948, pp. 376-379.
When photographic gelatin is heated above 245.degree. C., it gives up all retained water, exhibits a pyrolysis which frees some carbon, and is transformed physically from long helices to a shorter, random coil configuration typical of polymer materials. For reference, see The Science and Technology of Gelatin, Academic Press, 1977, pages 283-285.
Although it is possible to produce reflective metallic coatings of many types on substrates by vacuum sputtering or evaporation, silver is relatively unique in that it can also be produced and patterned by photographic techniques. Previously, a reflective silver laser recording medium was the subject of a prior patent application Ser. No. 012,235 by J. Drexler. In that application, a processed black silver emulsion was converted to a reflective recording medium by heating at a temperature in the range of 250.degree. C. to 330.degree. C. in an oxygen containing atmosphere until a shiny reflective appearance was achieved. The heating process appeared to break up the black filamentary silver grains into tiny grain segments of a few hundred angstroms. Over a period of minutes the heat and oxygen combined to create the surface reflective component of silver which is more concentrated at the surface and decreases monotonically into the body of the converted emulsion without creating a clearly defined layer of silver. It is believed that the conversion process includes the creation and then decomposition of silver oxide. High contrast digital-data recordings with reflective contrast variations of .+-.20% can be accomplished with a 5 milliwatt laser beam 0.8 microns in diameter and with a pulse length of 100 nanoseconds.
A silver diffusion transfer negative reflective photographic process leading to a reflective data storage medium without the use of a thermal processing step was the subject of prior patent application Ser. No. 55,270 by E. W. Bouldin and J. Drexler. The reflective electrically non-conducting data storage and laser recording medium was made from a commercially available photosensitive silver-halide emulsion by a silver diffusion transfer negative process and relies on the high refractive index of the silver-gelatin composite at the emulsion surface to create the reflectivity. In that application a well defined layer of reflective silver gelatin was created at a surface of the silver-halide emulsion by a latent image formation followed by a special monobath development treatment involving a small amount of chemical development and chemical diffusion transfer of the silver complexes and solution physical development of the latent image. High contrast digital-data recordings with reflective contrast variations of .+-.40% were accomplished with a 13 milliwatt laser beam 0.8 microns in diameter and with a pulse length of 100 nanoseconds. These reflective contrast variations appeared to be associated with local variations in reflectivity owing to local variations of silver density within the gelatin layer.
Silver diffusion transfer negative and reversal processes have been described in the patent literature. In U.S. Pat. No. 3,464,822 Blake discloses a silver diffusion transfer reversal process for creating electrically conducting silver images for the fabrication of printed circuit boards. That invention, in turn, is based upon silver diffusion transfer process inventions of the reversal type, leading to black non-reflective and non-conductive images, one example being U.S. Pat. No. 2,500,421 by E. H. Land. The silver diffusion transfer reversal process forms the basis of direct positives by the Polaroid Land process of Polaroid Corporation and the Gevacopy and Copyrapid processes of Agfa-Gevaert. These reversal processes should be distinguished from the silver diffusion negative process. One such process leading to black non-reflecting and non-conducting images, is described in U.S. Pat. No. 3,179,517 by Tregillus.
An object of the invention was to devise a non-toxic, highly sensitive reflective DRAW laser recording and data storage medium which may be manufactured without the use of a vacuum system and on a continuous basis and which may be used to record low-reflective spots in a reflective field with relatively low energy laser pulses. Another object was to devise a reflective laser recording and data storage medium of high recording sensitivity which permits the pre-recording of control indicia and certain data base data by photographic means to facilitate the use of discs or plates in both the recording apparatus and the playback apparatus. Another object was to permit replication of optically recorded media by photographic contact printing, readable in reflection or transmission. Another object was to devise a high sensitivity laser recording and data storage medium which could be fabricated from commercially available photoplates. A more specific objective was to devise a method of achieving a higher recording sensitivity and reduced variations in reflective contrast than achieved in recording media described in co-pending patent applications Ser. Nos. 012,235 and 55,270 while retaining the valuable attributes of the laser recording materials disclosed in those applications.